In implementing a variety of RF communication systems such as high frequency terrestrial communication systems, broadcast transmitting antenna systems, and particularly cellular and land mobile radio systems, a major obstacle is the provision of optimum RF coverage in difficult, confined areas such as tunnels, subways, depressed roadways, buildings, etc., where radiating RF energy is substantially blocked in both the transmitting and receiving directions. The problem is typically approached by using some form of directional antenna or radiating cable system for obtaining adequate coverage throughout the confined or blocked areas.
Tunnels represent a particularly difficult environment for RF coverage because of the natural blockage presented to high frequency radio signals by the concrete, earth and steel disposed throughout the confines of the tunnels. In small tunnels, optimum RF coverage can be obtained by using directional antennas which are disposed at the tunnel portals and appropriately adjusted. A major disadvantage associated with the use of directional antennas, however, is that any large deviations in the tunnel geometry such as bends or turns can result in significant signal loss. In addition, such an approach is also highly prone to blockage from large vehicles such as trucks and trailers, and is impractical for use with extended enclosed areas such as long tunnels or subways.
RF coverage for very long tunnels or subways has generally been provided by using distributed antenna systems using low-loss coaxial cable or fiberoptic arrangements for signal distribution, and a series of antennas which are fed by taps attached to the transmission line. The problem with this approach is that, since high frequency radio signals are completely confined to the tunnel due to the natural RF blocking properties of the tunnel composition, the radiation pattern of point source radiators, such as antennas, can cause reflections resulting in serious multi-path fading and signal attenuation and nulling. Further, it becomes necessary that the distributed antennas be carefully tuned in order to maintain the necessary isolation among the plurality of system antennas.
A conventional approach to providing optimum signal distribution and coupling of RF energy within tunnels or subways without employing discrete antennas has been the use of radiating cables. As opposed to standard coaxial cables which transmit electrical signals to and from a generating station to some form of antenna from where the signals are radiated, radiating cables themselves function as continuous antennas--electrical or radio signals are transmitted directly from the cables rather than from an antenna. Such radiating or "leaky" coaxial cables form efficient and economic sources for transmitting radio frequency signals in a variety of applications such as 2-way mobile radio, radio paging and other localized broadcast services in applications involving extended underground installations. The radiating cable approach becomes indispensable in applications such as railways, subways, mines and tunnels, where conventional centralized VHF and UHF communication systems are not practical.
In radiating cables, slots are provided in the typically corrugated metallic outer conductor of a coaxial cable to allow a controlled portion of the transmitted RF signals to radiate along the entire length of cable. Conversely, signals transmitted near the cable will couple into these slots and be carried along the cable back to the associated base station receiver. Because radiating cables can be designed for broadband operation, it is possible for a single length of radiating cable to simultaneously handle two or more communication systems. Since the cable can conveniently be routed wherever signal coverage is needed, radiating cables can be adapted for operation in areas of any form-factor, open or enclosed, which require localized coverage.
Because of the above advantages, the radiating cable approach is increasingly being used in tunnel coverage systems where two-way RF communications between base station transceivers and portable or mobile communication units is essential. In these systems, signals transmitted to and from base stations to mobile communication units within a tunnel are distributed through a series of amplifiers over a plurality of radiating cable lengths. Because of the finite coaxial attenuation factor of the radiating cable, it becomes essential to use signal amplifiers at periodic intervals. The amplifiers are typically bi-directional in order to accommodate and amplify signals moving along the cable in both directions, and provide a fixed amount of gain for each RF carrier signal in each signal direction. Since most communication systems utilize multiple carriers for normal operations, it becomes necessary to use Class-A linear amplifiers capable of providing wide-band gain.
Since ideal linear amplifiers cannot be realized in practice, two-way communication systems using the radiating cable approach are subject to signal distortion resulting from the finite amount of intermodulation distortion generated due to component non-linearities when amplification occurs at high power levels. Out-of-band intermodulation does not present a serious problem because it can usually be filtered out of the system. However, in-band intermodulation caused by intermodulation frequencies resulting from undesirable combinations of sinusoidal components of input frequencies can substantially affect system performance since it represents a noise source to a receiving unit which cannot be filtered.
The intermodulation distortion problem is further compounded when a plurality of amplifiers are cascaded in order to achieve the high transmission signal levels necessary to combat coaxial loss resulting from use of radiating cable across long tunnels or subways. When a chain of cascaded amplifiers exists, the intermodulation produced by one stage is amplified by the gain factor of each succeeding stage, which in itself generates further intermodulation components. In effect, in-phase addition of frequency components at each stage of amplification in a cascaded amplifier system also realizes a cascading or compounding of the intermodulation components. The end result is serious degradation of the RF signals to an unmanageable level.
Accordingly, in previously known systems, the implementation of radiating cable based two-way RF communication has severe restrictions regarding the practical number of amplifiers that may be used in any single chain.